Normal converters of automatic transmissions allow some slippage, even at cruising speeds. This is due to the fact that the only connection between pump and turbine of the converter is the transmission fluid.
To prevent this slipping action, and thus improve fuel economy, a number of converters are equipped with a lockup feature. When a lockup unit is actuated, the pump (impeller) and turbine are mechanically locked together. Some vehicle engines equipped with such converters are so designed as to lock the pump and turbine together so as to use efficiently the braking power of an engine when the vehicle engine is decelerated. In such a vehicle engine, the converter of an automatic transmission is unlocked to decrease engine loading so as not only to prevent engine stall but also reduce shocks when the speed of vehicle is decreased.
Meanwhile, for better fuel economy, there are known fuel injection systems controlled to temporarily shut off the injection of fuel when the vehicle slows down. Such a fuel injection shut-off control is terminated when the speed of vehicle engine reaches a predetermined or preselected or specified speed (which is called a fuel injection recovery speed in this specification) for the purpose of the prevention of engine stall.
In the vehicle engine which has such a fuel injection cut-off feature and is in cooperation with an automatic transmission with a torque converter through which the vehicle engine and a propeller shaft are cooperated even when the vehicle slows down, it becomes hard to make use of the braking power of engine and, furthermore, to make the injection of fuel shut off for a long period of time in comparison with vehicle engines equipped with manual transmissions because the vehicle engine rapidly slows down. This leads to a bad fuel economy.
In an attempt at overcoming such problems with such vehicle engines equipped with automatic transmissions in which a fuel cut control is carried out when the vehicle engine is decelerated, as is disclosed in, for example, Japanese Patent Unexamined Publication No. 61-184,269 entitled "Lockup control device for an automatic transmission used on a vehicle equipped with an engine with fuel cut control", laid open on Aug. 16, 1986, an automatic transmission is locked up when the vehicle engine operates within a speed range in which a fuel cut control is carried out while the vehicle engine is decelerated, to thereby maintain the vehicle engine and a propeller shaft in cooperation with each other so as not only to make use efficiently of the braking power of the vehicle engine but to maintain the vehicle engine operating at a speed above a fuel cut speed at which the injection of fuel is suspended in order to save fuel. Any such vehicle engine is adapted to unlock the automatic transmission immediately before the recovery of fuel injection in order to avoid shocks possibly generating due to a rapid development of engine power as a result of the recovery of fuel injection.
In practice, there is a delay of control from the provision of a lockup release signal till the automatic transmission is actually unlocked. Such a delay of lockup release control is caused due to the resistance of pressurized oil developed upon operating a control valve in a hydraulic power circuit for lockup means, such as a lockup clutch, of the automatic transmission. For compensating the delay of lockup release control, a speed of vehicle engine at which the automatic transmission is unlocked (which is called a lockup release speed in this specification) is usually set higher than a fuel injection recovery speed.
A serious problem in association with the vehicle engine is that, when a rapid deceleration takes place, the automatic transmission is still governed by the delay of lockup release control and the automatic transmission is often brought unlocked after the vehicle engine has lowered down below the fuel injection recovery speed. Accordingly, due to a rapid development of engine power, the vehicle engine will be subjected to shocks resulting in an engine stall.